the Session Initiation Protocol (SIP)

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

1. Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [3].

2. Background and Appropriateness

Message Waiting Indication is a common feature of telephone networks.
It typically involves an audible or visible indication that messages
are waiting, such as playing a special dial tone (which in telephone
networks is called message-waiting dial tone), lighting a light or
indicator on the phone, displaying icons or text, or some
combination.

Message-waiting dial tone is similar to but distinct from stutter
dial tone. Both are defined in GR-506 [11].

The methods in the SIP [1] base specification were only designed to
solve the problem of session initiation for multimedia sessions, and
rendezvous. Since Message Waiting Indication is really status
information orthogonal to a session, it was not clear how an IP
telephone acting as a SIP User Agent would implement comparable
functionality. Members of the telephony community viewed this as a
shortcoming of SIP.

Users want the useful parts of the functionality they have using
traditional analog, mobile, and PBX telephones. It is also desirable
to provide comparable functionality in a flexible way that allows for
more customization and new features. SIP Specific Event Notification
(RFC 3265 -- SIP Events) [2] is an appropriate mechanism to use in
this environment, as it preserves the user mobility and rendezvous
features which SIP provides.

Using SIP-Specific Event Notification, a Subscriber User Agent
(typically an IP phone or SIP software User Agent) subscribes to the
status of their messages. A SIP User Agent acting on behalf of the
user's messaging system then notifies the Subscriber each time the
messaging account's messages have changed. (This Notifier could be
composed with a User Agent that provides a real-time media interface
to send or receive messages, or it could be a stand-alone entity.)
The Notifier sends a message summary in the body of a NOTIFY, encoded
in a new MIME type defined later in this document. A User Agent can
also explicitly fetch the current status.

A SIP User Agent MAY subscribe to multiple accounts (distinguished by
the Request URI). Multiple SIP User Agents MAY subscribe to the same
account.

Before any subscriptions or notifications are sent, each interested
User Agent must be made aware of its messaging notifier(s). This MAY
be manually configured on interested User Agents, manually configured
on an appropriate SIP Proxy, or dynamically discovered based on
requested caller preferences [4] and registered callee capabilities
[5]. (For more information on usage with callee capabilities, see
Section 4.2)

3. Event Package Formal Definition

3.1. Event Package Name

This document defines a SIP Event Package as defined in RFC 3265 [2].
The event-package token name for this package is:

"message-summary"

3.2. Event Package Parameters

This package does not define any event package parameters.

3.3. SUBSCRIBE Bodies

This package does not define any SUBSCRIBE bodies.

3.4. Subscription Duration

Subscriptions to this event package MAY range from minutes to weeks.
Subscriptions in hours or days are more typical and are RECOMMENDED.
The default subscription duration for this event package is one hour.

3.5. NOTIFY Bodies

A simple text-based format is proposed to prevent an undue burden on
low-end user agents, for example, inexpensive IP phones with no
display. Although this format is text-based, it is intended for
machine consumption only.

A future extension MAY define other NOTIFY bodies. If no "Accept"
header is present in the SUBSCRIBE, the body type defined in this
document MUST be assumed.

The format specified in this proposal attempts to separate orthogonal
attributes of messages as much as possible. Messages are separated
by message-context-class (for example: voice-message, fax-message,
pager-message, multimedia-message, text-message, and none), by
message status (new and old), and urgent and non-urgent type.

The text format begins with a simple status line, and optionally a
summary line per message-context-class. Message-context-classes are
defined in [7]. For each message-context-class, the total number of
new and old messages is reported in the new and old fields.

In some cases, detailed message summaries are not available. The
status line allows messaging systems or messaging gateways to provide
the traditional boolean message waiting notification.

Messages-Waiting:

yes

If the Request-URI or To header in a message-summary subscription
corresponds to a group or collection of individual messaging
accounts, the notifier MUST specify to which account the message-
summary body corresponds. Note that the account URI MUST NOT be
delimited with angle brackets ("<" and ">").

Message-Account: sip:alice@example.com

In the example that follows, more than boolean message summary
information is available to the User Agent. There are two new and
four old fax messages.

Fax-Message:

2/4

After the summary, the format can optionally list a summary count of
urgent messages. In the next example there are one new and three old
voice messages, none of the new messages are urgent, but one of the
old messages is. All counters have a maximum value of 4,294,967,295
((2^32) - 1). Notifiers MUST NOT generate a request with a larger
value. Subscribers MUST treat a larger value as 2^32-1.

Voice-Message: 1/3 (0/1)

Optionally, after the summary counts, the messaging systems MAY
append RFC 2822 style message headers [9], which further describe
newly added messages. Message headers MUST NOT be included in an
initial NOTIFY, as new messages could be essentially unbounded in
size. Message headers included in subsequent notifications MUST only
correspond to messages added since the previous notification for that
subscription. A messaging system which includes message headers in a
NOTIFY MUST provide an administrator configurable mechanism to select
which headers are sent. Headers likely for inclusion are To, From,
Date, Subject, and Message-ID. Note that the formatting of these
headers in this body is identical to that of SIP extension-headers,
not the (similar) format defined in RFC 2822.

Implementations which generate large notifications are reminded to
follow the message size restrictions for unreliable transports
articulated in Section 18.1.1 of SIP [1].

Mapping local message state to new/old message status and urgency is
an implementation issue of the messaging system. However, the
messaging notifier MUST NOT consider a message "old" merely because
it generated a notification, as this could prevent another
subscription from accurately receiving message-summary notifications.
Likewise, the messaging system MAY use any suitable algorithm to
determine that a message is "urgent".

Messaging systems MAY use any algorithm for determining the
appropriate message-context-class for a specific message. Systems
which use Internet Mail SHOULD use the contents of the Message-
Context header [7] (defined in RFC 3458) if present as a hint to make
a context determination. Note that a composed messaging system does
not need to support a given context in order to generate
notifications identified with that context.

3.6. Subscriber Generation of SUBSCRIBE Requests

Subscriber User Agents will typically SUBSCRIBE to message summary
information for a period of hours or days, and automatically attempt
to re-SUBSCRIBE well before the subscription is completely expired.
If re-subscription fails, the Subscriber SHOULD periodically retry
again until a subscription is successful, taking care to backoff to
avoid network congestion. If a subscription has expired, new re-
subscriptions MUST use a new Call-ID.

The Subscriber SHOULD SUBSCRIBE to that user's message summaries
whenever a new user becomes associated with the device (a new login).
The Subscriber MAY also explicitly fetch the current status at any
time. The subscriber SHOULD renew its subscription immediately after
a reboot, or when the subscriber's network connectivity has just been
re-established.

The Subscriber MUST be prepared to receive and process a NOTIFY with
new state immediately after sending a new SUBSCRIBE, a SUBSCRIBE
renewal, an unsubscribe, a fetch, or at any other time during the
subscription.

When a user de-registers from a device (logoff, power down of a
mobile device, etc.), subscribers SHOULD unsubscribe by sending a
SUBSCRIBE message with an Expires header of zero.

3.7. Notifier Processing of SUBSCRIBE Requests

When a SIP Messaging System receives SUBSCRIBE messages with the
message-summary event-type, it SHOULD authenticate the subscription
request. If authentication is successful, the Notifier MAY limit the
duration of the subscription to an administrator defined amount of
time as described in SIP Events [2].

3.8. Notifier Generation of NOTIFY Requests

Immediately after a subscription is accepted, the Notifier MUST send
a NOTIFY with the current message summary information. This allows
the Subscriber to resynchronize its state. This initial
synchronization NOTIFY MUST NOT include the optional RFC 2822 style
message headers [8].

When the status of the messages changes sufficiently for a messaging
account to change the number of new or old messages, the Notifier
SHOULD send a NOTIFY message to all active subscribers of that
account. NOTIFY messages sent to subscribers of a group or alias,
MUST contain the message account name in the notification body.

A Messaging System MAY send a NOTIFY with an "Expires" header of "0"
and a "Subscription-State" header of "terminated" before a graceful
shutdown.

3.9. Subscriber Processing of NOTIFY Requests

Upon receipt of a valid NOTIFY request, the subscriber SHOULD
immediately render the message status and summary information to the
end user in an implementation specific way.

The Subscriber MUST be prepared to receive NOTIFYs from different
Contacts corresponding to the same SUBSCRIBE. (The SUBSCRIBE may
have been forked).

3.10. Handling of Forked Requests

Forked requests are allowed for this event type and may install
multiple subscriptions. The Subscriber MAY render multiple summaries
corresponding to the same account directly to the user, or MAY merge
them as described below.

If any of the "Messages-Waiting" status lines report "yes", then the
merged state is "yes"; otherwise the merged state is "no".

The Subscriber MAY merge summary lines in an implementation-specific
way if all notifications contain at least one msg-summary line.

3.11. Rate of Notifications

A Notifier MAY choose to hold NOTIFY requests in "quarantine" for a
short administrator-defined period (seconds or minutes) when the
message status is changing rapidly. Requests in the quarantine which
become invalid are replaced by newer notifications, thus reducing the
total volume of notifications. This behavior is encouraged for
implementations with heavy interactive use. Note that timely
notification resulting in a change of overall state (messages waiting
or not) and notification of newly added messages is probably more
significant to the end user than a notification of newly deleted
messages which do not affect the overall message waiting state (e.g.,
there are still new messages).

Notifiers SHOULD NOT generate NOTIFY requests more frequently than
once per second.

3.12. State Agents and Lists

A Subscriber MAY use an "alias" or "group" in the Request-URI of a
subscription if that name is significant to the messaging system.
Implementers MAY create a service which consolidates and summarizes
NOTIFYs from many Contacts. This document does not preclude
implementations from building state agents which support this event
package. One way to implement such a service is with the event list
extension [10].

3.13. Behavior of a Proxy Server

There are no additional requirements on a SIP Proxy, other than to
transparently forward the SUBSCRIBE and NOTIFY methods as required in
SIP. However, Proxies SHOULD allow non-SIP URLs. Proxies and
Redirect servers SHOULD be able to direct the SUBSCRIBE request to an
appropriate messaging notifier User Agent.

4. Examples of Usage

4.1. Example Message Flow

The examples shown below are for informational purposes only. For a
normative description of the event package, please see sections 3 and
5 of this document.

In the example call flow below, Alice's IP phone subscribes to the
status of Alice's messages. Via headers are omitted for clarity.

4.2. Example Usage with Callee Capabilities and Caller Preferences

The use of callee capabilities is optional but encouraged. If callee
capabilities are used, a messaging notifier MAY REGISTER a Contact
with an appropriate methods and events tag as shown in the example
below. To further distinguish itself, the messaging notifier MAY
also REGISTER as a Contact with the actor="msg-taker" tag. An
example of this kind of registration follows below.

5. Formal Syntax

The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC 2234 [6].

5.1. New Event-Package Definition

This document defines a new event-package with the package name:

message-summary

5.2. Body Format Syntax

The formal syntax for the application/simple-message-summary MIME
type is described below. The message-context-class production is
defined in Section 6.2 of RFC 3458 [7]. Note that all productions
described here are case insensitive.

6. Security Considerations

Message summaries and optional message bodies contain information
which is typically very privacy sensitive. At a minimum,
subscriptions to this event package SHOULD be authenticated and
properly authorized. Furthermore, notifications SHOULD be encrypted
and integrity protected using either end-to-end mechanisms, or the
hop-by-hop protection afforded messages sent to SIPS URIs.

Additional and privacy security considerations are discussed in
detail in SIP [1] and SIP Events [2].

7. IANA Considerations

7.1. SIP Event Package Registration for message-summary

Package name: message-summary

Type:

package

Contact:

[Mahy]

Published Specification: This document.

7.2. MIME Registration for application/simple-message-summary

MIME media type name: application

MIME subtype name: simple-message-summary

Required parameters: none.

Optional parameters: none.

Encoding considerations: This MIME type was designed for
use with protocols which can carry binary-encoded data.
Although the format of this MIME type is similar to RFC 2822,
it is not identical. (Specifically, line folding rules are
SIP-specific and included URIs can contain non-ASCII
characters.) Protocols which do not carry binary data
(which have line length or character-set restrictions
for example) MUST use a reversible transfer encoding
(such as base64) to carry this MIME type.

Security considerations: See the "Security Considerations"

section in this document.

Interoperability considerations: none

Published specification: This document.

Applications which use this media: The simple-message-summary
application subtype supports the exchange of message waiting and
message summary information in SIP networks.

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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.